This invention relates to fuel elements for nuclear reactors, and more particularly it deals with a novel method of production of a composite type cladding tube including a metallic liner of zirconium on the inner wall surface of a zirconium alloy tube.
Nuclear fuel elements for constituting the core of a power reactor designed, produced and operated nowadays each include nuclear fuel material contained in a cladding tube of high corrosion resistance, nonreactivity and good thermal conductivity. Such fuel elements are arranged in an array in lattice form at regular intervals to provide a nuclear fuel assembly, and a plurality of nuclear fuel assemblies of a suitable number form a conglomerate of a nuclear fission chain reactor type or a reactor core capable of carrying out a self-maintaining nuclear fission. The reactor core is placed in a pressure vessel through which coolant flows.
The cladding is used for various purposes. One of its two main objects is to prevent chemical reaction between nuclear fuel and coolant or between nuclear fuel and moderator. The second object is to prevent radioactive nuclear fission products, which are in part gaseous, from leaking into the coolant or moderator from the fuel. Cladding material usually used is stainless steel or zirconium alloys.
When a certain type of metal or alloy is used as the cladding material for producing and operating nuclear fuel elements, the cladding material undergoes mechanical and chemical reactions under specific conditions. Various problems have been caused from this fact. Zirconium and its alloys are excellent nuclear fuel cladding materials under steadystate operational conditions. This quality is attributed to the fact that zirconium and its alloys have a small neutron-absorption cross section and are strong, ductile, very stable and nonreactive in the presence of pure water or steam usually used as coolant and moderator in a nuclear reactor at a temperature below about 400.degree. C.
However, cladding materials have a problem because embrittlement occurs due to the interaction between the nuclear fuel, cladding and the nuclear fission products produced by the nuclear fission reaction, with a result that crack formation tends to occur in the cladding. It has been ascertained that this behavior of the cladding materials is promoted by mechanical stresses that are produced locally in the cladding due to the difference in thermal expansion between the fuel and cladding. During operation of a nuclear reactor, nuclear fission products are released from the nuclear fuel due to fission reaction and exist on the surface of the cladding. In the presence of specific nuclear fission products such as iodine and cadmium etc. stress corrosion cracking occurs due to the action of local stresses and strain.
As disclosed in U.S. Pat. No. 4,200,492 and U.S. Ser. No. 522,856 dated Nov. 11, 1974, proposals have been made to provide various types of metallic liners between the fuel and cladding to avoid the aforesaid problem. Of all the metallic liners that have hitherto been proposed, the liner which is believed to have the brightest prospect is a coating of zirconium of a suitable purity provided as a lining to the inner wall surface of a zirconium alloy cladding tube. The thickness of the zirconium liner is about 5-30% that of the cladding tube. Zirconium which is better able to remain in a soft state than zirconium alloys during irradiation by neutrons has the effect of reducing local strain in the nuclear fuel element, to thereby protect the cladding tube from stress corrosion cracking and liquid metal embrittlement. An additional feature of zirconium is that it does not involve the problems of neutron capture penalty, heat transfer penalty and incompatibility of material. Besides zirconium, nickel and copper may also be used to provide a metallic liner.
This type of composite cladding type is usually produced by the following process. As shown in FIG. 1, a zirconium ingot for producing a metallic liner and a zirconium alloy ingot for producing cladding material are produced by the melting of briquettes. Then the ingots are subjected to heat treatment, forging and machining to form a second hollow billet of zirconium and a first hollow billet of zirconium alloy. The second hollow billet is inserted in the first hollow billet, and they are formed into a unitary structure either by explosion joining or diffusion joining, to provide a composite billet. The composite billet produced in this way is extruded by a usual hot extrusion process at a high temperature of about 550.degree.-750.degree. C. Then the extruded composite tube is subjected to a usual tube making process by use of cold rolling, to provide a finished cladding tube of the desired dimensions.
It is necessary that a composite type cladding tube produced by the aforesaid process have the thickness of its zirconium liner controlled to the desired value and have its zirconium liner completely joined over its entire area to the cladding material in metallurgical bonding. To this end, the second hollow billet (inner tube) of zirconium should be unitary with the first hollow billet (outer tube) of zirconium alloy when they are formed into a composite billet, and the composite billet should be deformed in a unitary condition when subjected to extrusion and tube making working. To obtain a unitary structure in a composite billet, there is adopted herein a process disclosed in U.S. patent application filed Apr. 15, 1981 and assigned Ser. No. 254,297, claiming Convention priority based on Japanese Patent Application No. 50748/80.
It has been desired that, in order to obtain a cladding tube for nuclear fuel elements in which tube a zirconium liner is in a uniform and good joinder in the whole interface, there is realized a method of producing composite type cladding tube comprising the above-mentioned process for obtaining a composite billet of a unitary structure.
This invention has as its object the provision of a novel method of production of a cladding tube of a composite type.
One outstanding characteristic of the invention is that the method according to the invention comprises the step of mechanically joining a second hollow billet (inner tube) of zirconium to a first hollow billet (inner tube) of zirconium alloy. That is, following insertion of the inner tube of zirconium into the outer tube of zirconium alloy, a resilient member serving as a pressing medium having a greater axial length than the inner tube is inserted in the inner tube. Then an axial compressive load is applied to the pressing medium from outside, to bring the outer surface of the inner tube into intimate pressure contact with the inner surface of the outer tube. The feature of the invention is that the use of a resilient member as a pressing medium enables a force to be exerted uniformly on the inner tube both axially and circumferentially, because the pressing medium behaves in the same manner as fluid. Also, control of the force can be readily effected so that it is possible to achieve pressure bonding of high reliability without causing deformation or any change in the thickness of the inner tube of zirconium.
The present invention is based on the discovery made in the process of expanding a tube which is to be described hereinafter.
A load for expanding a tube can be obtained by the following equation: EQU W=PA.sub.r /.eta. (1)
where
W: tube expanding load (kg). PA1 P: tube expanding pressure (kg weight/cm.sup.2). PA1 A.sub.r : cross-sectional area of tube expanding rubber member (mm.sup.2). PA1 .eta.: tube expanding efficiency (0.85-0.9). PA1 Z.sub.r : thickness of inner member of zirconium. PA1 Z.sub.ry : thickness of outer member of zirconium alloy.
FIG. 3 shows the results of tests in which there is shown the relation between bonding strength measured by drawing and the tube expanding pressure applied to the inner tube of zirconium according to equation (1). It will be seen that the higher the tube expanding pressure, the higher becomes the bonding strength. The bonding strength measured by drawing refers to a load required for separating the inner tube from the outer tube of the composite tube after the composite is produced by the tube expansion. In producing a composite fuel cladding tube, it is preferable to increase the bonding strength because the composite billet obtained is subjected to hot extrusion and tube contraction working at a working rate of over 80%. Experiments were conducted by the inventors to determine the lower limit of the preferable bonding strength. The results obtained show that the bonding strength of a composite cladding billet should be preferably within the range indicated by a hatch in FIG. 3.
Further, it has been found out by the inventors that in order to control the thickness of a zirconium liner of a composite type cladding tube to a desired level, the ratio of the thickness of the inner member of zirconium to that of the outer member of zirconium alloy at the time of tube expansion plays an important role. The thickness ratio is expressed by the following equation: ##EQU1## where t: thickness ratio.
Experiments were conducted regarding tube expansion by varying the ratio of thickness of the inner member of zirconium to that of the outer member of zirconium alloy, and the final thickness ratio was obtained by producing a composite type cladding tube of the final stage. The results show that the thickness ratio at the final stage is lower by 5-10% than at the time of initial tube expansion. This is attributed to the fact that zirconium is easier to work or deformed than zirconium alloys. Thus it has been ascertained that in order to obtain the desired thickness for the zirconium liner, the thickness ratio should be increased by 5-10% at the time of the tube expansion as compared with the desired thickness ratio of the cladding tube obtained as a completed product.
Another outstanding characteristic of the invention is that the interface between the inner and outer member of the composite billet is joined and hermetically sealed at opposite ends of the billet. Joining and sealing of the opposite ends of the cladding billet is achieved by electron beam welding, preferably by electron beam welding in high vacuum. By subjecting the composite type cladding billet to electron beam welding to close the opposite ends of the inner and outer tubes, and inner and outer tubes are rendered unitary with greater precision and no residual air is present between the surfaces of the two tubes contacting each other, so that joining of the two tubes is not prejudiced in any way by oxidation that might otherwise occur when hot extrusion is subsequently performed. In the electron beam welding, there can be minimized the degree of thermal effect caused by the welding as compared with other welding techniques, so that the grain growth of zirconium in the thermally affected zones into coarse grains is prevented thereby preventing grain growth which might adversely effect the subsequent working and the characteristics.
A still another outstanding characteristic of the invention is that it is possible to control the thickness of the zirconium liner of the composite type cladding tube. As described hereinabove, it is necessary to control the thickness of a zirconium liner to the desired value, in order to produce a composite type cladding tube of high reliability in performance. This control is effected according to the invention by polishing the inner surface of the cladding tube in the final finishing step.
Polishing of the inner surface may be carried out either mechanically by using a sand blast or through chemical polishing by using a mixture of nitric acid and fluoric acid or by a combination of the two processes. By suitably controlling the conditions (pressure, flow velocity, flow rate, time, etc.) of operation, it is possible to optimally control the thickness of the inner wall surface of the zirconium liner by varying the amount of polishing of the inner surface. By performing polishing of the inner surface of the cladding tube, it is possible to obtain a smooth surface on the inner wall surface of the zirconium inner tube.
By virtue of the aforesaid outstanding characteristics, the present invention is capable of providing a method of production of a composite type cladding tube having an increased precision in the thickness of the zirconium liner and an improved joining of the zirconium liner to the cladding over the entire area as compared with composite type cladding tubes provided by methods of the prior art.
Additional objects, features and advantages of the invention will become apparent from the description set forth hereinafter when considered in conjunction with the accompanying drawings.